Memory Model

Fixed memory allocation

Introducing StaticJsonBuffer

ArduinoJson uses a preallocated memory pool to store the object tree, this is done by the StaticJsonBuffer class.

Before using any function of the library you need to create a StaticJsonBuffer. Then you can use this instance to create arrays and objects, or parse a JSON string.

StaticJsonBuffer has a template parameter that determines its capacity. For example, the following line creates a StaticJsonBuffer with a capacity of 200 bytes:

StaticJsonBuffer<200>jsonBuffer;

The bigger the buffer is, the more complex the object tree can be, but also the more memory you need.

How to determine the buffer size?

So the big question you should have in mind right now is How can I determine the size?.

There are basically two approaches here:

either you can predict the content of the object tree,

or, you know how much memory is available.

In the first case, you know some constraints on the object tree. For instance, let’s say that you know in advance (and by that I mean “at compilation time”) that you want to generate an object with 3 values, one of them being an array with 2 values, like the following:

{"sensor":"gps","time":1351824120,"data":[48.756080,2.302038]}

To determine the memory usage of this object tree, you use the two macros JSON_ARRAY_SIZE(n) and JSON_OBJECT_SIZE(n), both take the number of elements as an argument.
For the example above, it would be:

In the second case, let’s say you dynamically generate a JSON object tree of a random complexity so you can’t put a limit based on that. But on the other hand, you don’t want your program to crash because the object tree doesn’t fit in memory.
The solution here is to determine how much memory is available, or in other words how much memory you can afford for the JSON object tree.

Why choose fixed allocation?

This fixed allocation approach may seem a bit strange, especially if you are a desktop application developer used to dynamic allocation, but it makes a lot of sense in an embedded context:

the code is smaller

it uses less memory

it doesn’t create memory fragmentation

it is predictable

Don’t forget that the memory is “freed” as soon as the StaticJsonBuffer is out of scope, like any other variable. It only holds the memory for a short amount of time.

Dynamic memory allocation

This library also supports dynamic memory allocation.

However, usage of this memory model make sense only for devices with more that 10KB or RAM

To switch to dynamic memory allocation, simply replace:

StaticJsonBuffer<N>jsonBuffer;

by

DynamicJsonBufferjsonBuffer;

Memory is released in DynamicJsonBuffer’s destructor, so you don’t have to do anything special.

Object size on ESP8266 if JsonFloat is changed to double

Where to go next?

In the ArduinoJson ebook, the chapter “Inside ArduinoJson” explains how StaticJsonBuffer and DynamicJsonBuffer work, and how to choose between them.

The book also contains a quick C++ course to catch up with memory management in general. It explains the differences between “stack,” “heap” and “global” memories.
It also debunks wrong assumption that memory management is done by the developer.